ci: use new Alpine 3.21 for Alpine-based build

Alpine 3.21 was released eleven days ago. See
<https://www.alpinelinux.org/posts/Alpine-3.21.0-released.html>.

.github/workflows/alpine.yml

@@ -5,7 +5,7 @@ on: push
 jobs:
   alpine:
     runs-on: ubuntu-24.04
-    container: alpine:latest
+    container: 'alpine:3.21'
     steps:
       # Git is required so that actions/checkout does a proper Git checkout.
       - name: Install Git in container